SELECT CASE
WHEN AGE BETWEEN 1 AND 10 THEN '1-10'
WHEN AGE BETWEEN 11 AND 20 THEN '11-20'
WHEN AGE BETWEEN 21 AND 30 THEN '21-30'
WHEN AGE BETWEEN 31 AND 40 THEN '31-40'
ELSE '40+'
END AS AGE_GRP,
Count(1) as Cnt
FROM yourtable
GROUP BY CASE
WHEN AGE BETWEEN 1 AND 10 THEN '1-10'
WHEN AGE BETWEEN 11 AND 20 THEN '11-20'
WHEN AGE BETWEEN 21 AND 30 THEN '21-30'
WHEN AGE BETWEEN 31 AND 40 THEN '31-40'
ELSE '40+'
END

If you don't want to repeat the CASE statement in GROUP BY then use this

SELECT AGE_GRP,
Count(1) AS cnt
FROM (SELECT CASE
WHEN AGE BETWEEN 1 AND 10 THEN '1-10'
WHEN AGE BETWEEN 1 AND 10 THEN '11-20'
WHEN AGE BETWEEN 1 AND 10 THEN '21-30 '
WHEN AGE BETWEEN 1 AND 10 THEN '31-40'
ELSE '40+'
END AS AGE_GRP
FROM yourtable) A
GROUP BY AGE_GRP